Printers such as inkjet printers which print onto a variety of print media such as paper or film are well known. As well as accepting print media in a single sheet format, some printers also accept print media fed from a supply roll of print media, in other words a web of print media. Such a printer may be typically referred to as a web printer, being a printer that accepts a web of print media. Paper, vinyl, textiles, fabrics, and others are examples of print media.
Relative movement between the print media and the print head enables print media coverage and image creation. A majority of billboards and banners having relatively large dimensions are printed on flexible print media. Such print media represent rolls or webs of flexible material that are up to five meters wide.
The feeding of a web of print media from a roll for a large format printer is typically undertaken by means of rollers, some of which induce the media movement and others change the media direction or form nips enabling media transportation. In the context of the present disclosure the term “nip” means the gap or width of the gap between two parallel rollers. The nip may have a desired width or the rollers may be in contact having a nip width equal to zero.
The print media is pulled from a roll that has a mechanism to provide tension (back-tension) in the media so as to reduce undulations or wrinkles in the media. In the context of the present application the term “back tension” means the force that keeps the substrate tensioned with respect to a drive roller. A difference in rotational speed between two or more rollers typically generates the back tension. Despite this back tension, undulations or wrinkles are sometimes formed before a nip and close to a location of one of the rollers, usually a tensioning roller. Small undulations are sometimes pulled into the nip between different rollers and reach a printing zone or region of the printer, degrading the quality of printed images. In some cases larger undulations may be pulled into the nip and irreparably damage the print media or even disrupt the printing process. One approach to these issues is to aim to reduce the number and size of the undulations by improving the accuracy of the printer, and/or using highly stable and relatively stiff print media, which complicates transportation and adds to the cost of both the printer and the print media.
International Patent Application Publication Number WO/2008/093157 discloses a method of tensioning and printing on a single wide format media roll comprising: applying a back tension to a web of print media being printed upon at a printing zone; and relaxing the back tension from time to time so as to allow undulations in the web downstream of a printing zone to relax out of the web prior to the web being wound onto a collection roller. This method is adequate when printing is performed on a single roll. However, when more than one roll of print media is simultaneously loaded on the printer, it is difficult to equally tension each of these rolls. The print quality on each web of print media may therefore be different and, in many cases, where the sections of the same billboard are printed on different webs (such as a large billboard) the assembly results in unwanted visible artifacts.